Linear solar receivers
for CSP
François Veynandt
Centre RAPSODEE
Ecole des Mines d’Albi
avec la contribution de
Jean Jacques BEZIAN
Summary

Overview
– Why linear concentration ?
– Various applications of linear systems

Linear receiver for
– parabolic trough
– FRESNEL concentrators
– CPVT

Linear receivers’ design issues:
example for Linear Fresnel Reflector
– Energy efficiency: thermal transfers, losses
– Development trend
2
Summary

Overview
– Why linear concentration ?
– Various applications of linear systems

Linear receiver for
– parabolic trough
– FRESNEL concentrators
– CPVT

Linear receivers’ design issues:
example for Linear Fresnel Reflector
– Energy efficiency: thermal transfers, losses
– Development trend
3
Why linear concentration ?

One axis concentration is more simple, only one axis
movement to follow the sun
 Maximum linear concentration on Earth is
46200 =210, 60 to 100 for commercial applications
4
Why linear concentration ?

Maximum temperature of black body is about 1150 K,
(835 to 950 K), good levels for industrial processes
5
Stagnation temperature as a function of concentration ratio C
Why linear concentration ?

Allows overheated steam at 500 °C (RANKINE cycle)
6
Optimal temperature as a function of concentration ratio C
Various applications

Solar power plants
Andasol
Puerto Errado
7
Various applications

Steam production for industrial processes
 Solar assisted heating and cooling
 Solar cogeneration (heat and power)
 Linear CPVT
8
Various applications : small sizes
Two axis concentrators

For small sizes, edge losses due to solar angle
 a second tracking is interesting:
– improves optical efficiency,
– only one tracking needs to be precise
9
Various applications : usually
One axis tracking

The most common solution
 For all applications: CSP, CPV, thermal
applications
10
Summary

Overview
– Why linear concentration ?
– Various applications of linear systems

Linear receiver for
– parabolic trough
– FRESNEL concentrators
– CPVT

Linear receivers’ design issues:
example for Linear Fresnel Reflector
– Energy efficiency: thermal transfers, losses
– Development trend
11
Two most common system:
Parabolic Trough (PT) power plant

Typical design: thermal oil and molten salt
storage
12
Two most common system:
Linear Fresnel Reflector (LFR) power plant

Typical design: direct steam generation,
without storage
13
Linear receivers design: considerations

Very long distances involved: (1 km/MW in a
PT plant)
 Depends on reflector geometry
 Goal: Achieve High Performance, Low Cost,
Reliability and Durability
14
Linear receivers designs: parameters

High optical efficiency
– tracking accuracy
– reflective components
– absorptive element

High thermal efficiency
– glass cover
– vacuum
– coating

Low cost
– Fabrication
– Transport
– Installation

High durability
– Corrosion resistance
– Low weight / wind resistance
15
Linear receiver for
parabolic trough



Experience of SEGS
plants since the 80’s
Mature design
Optimization on
details
16
Linear receiver for
parabolic trough: example


95 %: Schott PTR 70: 4 m long
Tube with selective coating
– 95 % solar absorption,
– 14 % IR emission 350 °C


In an evacuated glass tube
Mobile receiver
17
Linear receiver for
parabolic trough: example


More than 3 Gigawatts capacity equipped with
SCHOTT PTR® 70 receivers (over 1 Million receivers)
More than half of the market (over 50 CSP projects
around the globe)
18
Linear receivers for LFR collectors








Many designs exist:
each company has developed its own concept
Advantage: fixed receiver
Geometry: tube, V shape, trapezoidal cavity
Number of tubes: one, two or more
Heat transfer fluid: air, water/steam, organic fluid,
thermal oil, molten salt …
Secondary reflector or not?
Glass window (or not?)
Evacuated or not?
19
Linear receivers for LFR collectors
Examples

Various geometries
reference Negi et al. (1990, 1989), Gordon and Ries (1993) and
20
Abbas et al. (2012a,b).
Linear receivers for LFR collectors
Examples

Compact Linear Fresnel Reflector (CLFR) concept
reference Mills and Morrison (2000)

Mirror field optimization: etendue matched CLFR
21
reference Horta et al. (2011)
Linear receivers for LFR collectors
Examples

Trapezoidal receiver designs
22
reference Pye et al. (2003), Reynolds et al. (2004), Singh et al. (1999, 2010), Gordon and Ries (1993)
Linear receivers for LFR collectors
Examples

Receiver with secondary reflector:
Fresdemo receiver equiped with
photogrammetric measurement
foil on secondary reflector
Novatec Solar receiver with
Composed Parabolic Concentrator
(CPC)
reference Bernhard et al. (2008a,b), Selig and Mertins (2010)
23
Linear receivers for LFR collectors
Examples

New receiver with flatter secondary reflector
reference Grena and Tarquini (2011)
24
Linear receiver for CPVT




Cogeneration (power and heat) with PV cells cooled
by a fluid
Low temperatures (60 to 80 °C)
Average efficiency: 15 % (or more) for power, 50 %
(or less) for heat
More conductive transfers
25
Summary

Overview
– Why linear concentration ?
– Various applications of linear systems

Linear receiver for
– parabolic trough
– FRESNEL concentrators
– CPVT

Linear receivers’ design issues:
example for Linear Fresnel Reflector
– Energy efficiency: thermal transfers, losses
– Development trend
26
Linear receivers’ design issues:
Thermal transfer optimization

Best solar energy collection
 Least thermal losses
 Depends on:
–
–
–
–
The level of temperature
The fluid (air, water …)
The solar angle aperture
The flux map …
27
Linear receivers’ design issues:
Thermal transfers

Radiative transfers
–
–
–
–

Optical properties of selective coating
Net incident solar flux
Infra red emission (in the cavity)
Infra red emission (external losses)
Convective transfers
– In the tube (heat collection)
– In the cavity
– External losses

Conductive transfers, most often
negligible, except for the tube
28
Linear receivers designs
Diagram of thermal transfers

An example of the various thermal
transfers
29
- Radiative heat transfer
Selective coating





Absorber optical properties
Not suitable without glazing
Temperature range: - 70 °C, + 540 °C
Absorption: solar spectrum
Emission: black body at 400 °C
2 layers 3 layers 4 layers 5 layers 6 layers
Thickness
800 nm
900 nm
Absorption
0.87
0.90
0.91
0.91
0.92
Emission
0.22
0.23
0.23
0.24
0.24
30
- Radiative heat transfer
Incident flux map


Depend on the concentrator optical efficiency:
tracking and quality of the optical components
Non homogeneity in the flux distribution
– Over heated lines (and problem on the durability of coating)
– Impact on the fluid temperature (heat exchange and local
vaporization)
31
- Radiative heat transfer
Incident flux map

Results from
simulations using
EDStar, Monte Carlo
based radiative heat
transfer simulation tool
sun
Receiver
Mirrors
32
- Radiative heat transfer
Incident flux map

Variability with
– date of the year
– hour of the day
– optical efficiency

of:
– Total power
collected
– Homogeneity of
flux distribution
=> Improve
design for better
efficiency and
durability
33
- Receiver energy balance
Infra red exchanges
  T
4
New repartition between internal
surfaces: best homogeneity
 External losses

 Depends
on local conditions:

– Emissivity of surfaces,
– Temperature of surfaces (heat balance)
– Equivalent sky temperature
– Equivalent environment temperature
34
- Receiver energy balance
Convection in the tube
  hST

Collection of solar heat by a fluid
 Depends on the fluid (liquid, gas or 2 phases
flow), the temperature, the pressure …
 local conditions
 Various

h  Nu /D is given by various correlations,
depending on Reynolds number
 For example : Colburn :

35
- Receiver energy balance
Fluid Mechanics in the tube: Pressure drop
With  roughness (0.03 mm)
 Colebrook correlation

Linear receiver are long, each loop may
exceed 1 km
=> Pumping power is important to consider

38
- Receiver energy balance

Convection in the cavity
If the cavity is not evacuated
 Natural convection: h depending of Grashof number

Simplified hypothesis
39
- Receiver energy balance
Results

Temperature profiles along the
receiver pipe with air as HTF
40
- Receiver energy balance
Results

Temperature profiles along the
receiver pipe with water/steam
41
Linear receivers’ design issues:
Over heating of the secondary reflector

Good reflector (95 %), very bad emitter (1 %)
 In the higher part of the cavity (bad convective
transfer)
 Back insulation
=> Very high temperatures and deformations
42
=>Thermal efficiency of the receiver

Efficiency of the collector is the ratio between the heat
collected and the DNI x mirror area. It depends on:
– the optical efficiency of the concentrator (50 %)
– the thermal efficiency of the receiver (80 %): heat collected divided by
solar flux absorbed by the receiver

Losses are mainly:
– radiative losses: IR,
– convective losses: free or forced (wind) convection: from 5 to 50 W/m2K
43
Development trends of Linear
Fresnel Reflector

State of art:
– non-evacuated steel tubes (ex. Areva)
• suitable for 180-300°C (up to 480°C)
• significant losses over 400°C
– Direct Steam Generation
• +: saves an expensive heat exchanger
• +: easier operation and maintenance
• -: only short time storage

Towards higher temperatures:
– Evacuated pipes with secondary reflector
(demonstrated 520°C superheated steam ex. SuperNova, Novatec)
– Limits:
• optical efficiency for higher concentration
• Materials’ reliability

Towards base load:
– Molten salt as Heat Transfer Fluid and storage
44
Conclusions

Very long component of the plant (50 km for a
50 MW PT plant): /!\ cost, efficiency
 Suitable for many industrial uses
 Thermal efficiency very important
– Optical efficiency: Selective coating for high temperature
– Thermal efficiency: Evacuated tubes: expensive, efficient

Main receiver techniques:
– Mature evacuated pipe for PT
• most commercial CSP power plants today
– More opened subject for LFR
• towards base-load: evacuated tube, for high temperature
operation, with molten-salt as HTF and thermal storage
– Other solutions: cheaper, less efficient and not entirely
mature, but with potential for improvement
45

Zhu, G., Wendelin, T., Wagner, M. J., &
Kutscher, C. (2014). History, current state, and
future of linear Fresnel concentrating solar
collectors. Solar Energy, 103, 639–652.
doi:10.1016/j.solener.2013.05.021
 Cau, G., & Cocco, D. (2014). Comparison of
Medium-size Concentrating Solar Power Plants
based on Parabolic Trough and Linear Fresnel
Collectors. Energy Procedia, 45, 101–110.
doi:10.1016/j.egypro.2014.01.012
46